1. Field of the Invention
The present invention relates to a digital receiver, more particularly to a digital receiver for receiving and demodulating and a method of receiving and demodulating digital signals transmitted using various carrier frequencies.
2. Description of the Related Art
The Digital Video Broadcasting (DVB) organisation has specified a set of standards for encoding, modulation and transmission of digital TV channels. These digital channels offer advantages over conventional analogue TV channels in terms of greater bandwidth and power efficiency as well as robustness to interference and noise. The standards are being adopted in Europe and in other markets world-wide. The standards have been developed for cable (DVB-C as defined in ETS300 429 digital broadcasting systems for television sound and data services; framing structure, channel coding and modulation for cable systems), satellite (DVB-S as defined in ETS300 421 digital broadcasting systems for television sound and data services; framing structure, channel coding and modulation for 11/12 GHz satellite services) and terrestrial (DVB-T as defined in ETS300 744 digital broadcasting systems for television sound and data services; framing structure, channel coding and modulation for digital terrestrial television) digital television systems.
The DVB specify the various source coding and transmission methods used. Systems first compress the digital video using the MPEG-2 algorithm as defined in ISO/IEC 13818-2 Generic Coding of Moving Picture and Associated Audio:video. This allows many more channels to be carried in the same bandwidth. The compressed video stream is then coded using forward error correction (FEC) techniques. This allows the system to cope with noise and interference introducing bit errors into the bit stream. The error-protected stream is then timemultiplexed up with other compressed video and audio streams and other data into an MPEG-2 transport stream as defined in ISO/IEC 13818-1 Generic Coding and Moving Picture and Associated Audio:systems. This transport stream is then converted into a sequence of modulated symbols for transmission over the broadcast channel.
The mapping of the bit stream into symbols depends upon the modulation system used. Presently different modulation schemes are used for each type of system. Hence, QAM (Quadrature Amplitude Modulation) is used for cable systems, QPSK (Quadrature Phase Shift Keying) for satellite systems and COFDM (Coded Orthogonal Frequency Division Modulation) for terrestrial systems.
The digital TV receiver tunes to the digital TV broadcast channel and demodulates the received symbols into a bit stream. The bit stream is error corrected and decompressed to allow the recovery and display of the transmitted pictures. The digital TV is considerably more complex and therefore expensive than conventional analogue TV receivers. This is mainly due to the complex silicon devices needed to demodulate, decode and decompress a received digital signal into TV picture frames.
The increased cost can be offset against improved efficiency, functionality and interactivity offered by digital TV systems. However, recreation of some features of analogue receiver systems could significantly increase the cost of a digital television receiver and hence impede widespread adoption of digital TV receivers.
It is expected that users of digital TV receivers will expect those receivers to have at least the same functionality and features as the previous analogue receiver systems. This is particularly true in the case of integrated digital television systems having a number of different parts, such as at least a display and a recording means.
The present application recognises for the first time particular problems where a digital receiver system attempts to duplicate the functions of an analogue receiver system having a number of independent receivers. In particular, to produce the function of picture-in-picture (PIP) or to be able to record one television program whilst displaying a different television program, a digital system will require two independent digital receivers.
As mentioned above, the complexity of the demodulation process used in digital TV systems makes the digital TV receivers expensive. The provision of two or more receivers in a digital TV system will significantly increase the cost of the overall system.
According to the present invention, there is provided a digital receiver for receiving and demodulating a plurality of digital signals transmitted with different respective carrier frequencies, comprising:
a plurality of tuners, each for down converting a respective carrier frequency signal to a respective base-band signal and then digitising the respective base-band signal and each having a respective output at which the respective digitised base-band signal is provided; wherein
a demodulator is provided to receive the respective outputs of each of the plurality of tuners for demodulating the digitised base-band signals in a timed division manner.
According to the present invention there is also provided a method of receiving and demodulating a plurality of digital signals transmitted with different respective carrier frequencies comprising:
down converting carrier frequency signals to respective base-band signals and digitising the respective base-band signals; and
providing a common demodulator for demodulating the respective digitised base-band signals in a time division manner.
In this way, the expensive and complex demodulation process is carried out in a single unit for two or more different tuning sections. Hence, with only one demodulation section, a digital television system can tune to one channel for a main picture and another channel for a picture-in-picture. Similarly, the system can tune to a separate channel for recording a program.
Preferably, each tuner is independently tunable to allow independent selection of the two or more channels chosen by the user.
Each of the plurality tuners may additionally be provided with functions of amplifying and filtering the down converted base-band signal and the demodulator may provide separate respective automatic gain control and automatic frequency control to each of the plurality of tuners. In this way, each tuner can provide an output signal which is appropriate for use by the demodulator irrespective of the nature of the particular signal which the tuner receives. The gain, filtering and frequency control of each tuner may be independently varied to provide the same levels of output for the demodulator.
Preferably, each respective digitised base-band signal is provided to a separate input of a multiplexer, such that the demodulator may selectively demodulate the signals received at each input. In this way, the demodulator may demodulate signals from the tuners in a time division manner. The demodulator itself may have two inputs and internally be provided with a multiplexing function. Alternatively, the multiplexer may be provided as part of other control circuitry connecting the tuners to the demodulator. In this way, the demodulator is provided with a single input comprising time division multiplexed signals from both of the tuners.
Preferably, a respective buffer is provided at the output of each of the plurality of tuners, each buffer being sufficient to hold at least one received symbol. In this way, signals output from the tuners may be read into the respective buffers at a standard rate, but output at an increased rate so as to allow time division multiplexing.
The demodulator section can operate under such a clock frequency as to allow demodulation of received symbols at twice or more than the normal rate since the demodulation process clock rate can be made independent of the received symbol clock rate.
The demodulator section can also independently process received symbols from two or more sources and can save the state of a process on a particular source of received symbols whilst processing other sources.
Preferably, the buffers are dual port FIFOs and the demodulator provides an output clock to the buffers so as to receive the respective digitised base-band signals at at least twice the rate at which they are stored in the buffers so as to enable at least two signals to be demodulated in a time division manner.
Preferably, the invention may be incorporated in a digital television receiver for receiving and demodulating signals from at least one of a cable, a satellite and a terrestrial source.
The digital television receiver may be incorporated into a television apparatus having a display for displaying images produced from at least one of the demodulated signals.
The apparatus may comprise means for producing on the display a main picture from one of the at least one of the demodulated signals and a picture-in-picture image from another of said least one of the demodulated signals.
The apparatus may also comprise a data recorder for selectively recording the at least one or another of the demodulating signals.
In this way, two television images may be displayed simultaneously. Additionally, a television image may be recorded independently of one or more television images being displayed on the television display.